This invention relates generally to methods for forming composite structures, and, more specifically, to a method for forming composite structures utilizing an impervious flexible film which allows the application of a uniformly distributed pressure to a lay-up, and which tends to prevent the flow of resin out of the lay-up during a curing cycle.
It is well-known to use a vacuum bag molding process to supply desired compression on a resin impregnated lay-up while heat curing a composite structure. To obtain a quality part, it is often necessary to remove all of the entrapped air from the lay-up being formed into the composite structure, or gases emitted by the resindue to the polycondensation which may occur during the heat curing, depending on the resin utilized. While most prior systems which have provided for the escape of entrapped air or volatiles have successfully accomplished the intended task, they have necessarily also provided paths for the escape of excessive amounts of resin from the lay-up during the during cycle.
Most prior art systems have used various layers of breather materials, including glass cloths, porous mats, etc., which have been placed in communication with both the lay-up and the vacuum source. As the viscosity of the resin within the lay-up is lowered as a result of an increase in temperature during the curing cycle, catastrophic resin flows can result, yielding low strength, out of tolerance parts, pluged-up vacuum systems, and the like.
One solution to the problem of excessive resin bleeding or squeeze-out during the curing of composites is found in U.S. Pat. No. 4,216,047, issued Aug. 5, 1980, and entitled "NO-BLEED CURING OF COMPOSITES", wherein strips are provided which purportedly allow air to be completely removed from the lay-up during the initial stages of the curing cycle, but which close off when resin starts to flow into the breather strips. Notwithstanding the advance set forth in the aforementioned patent, practical experience has shown that the provision of breather strips in communication with both the resin impregnated lay-up and the vacuum source still allows resin to undesirably escape the lay-up during the curing cycle.
Another method utilized to restrict the resin flow out of the lay-up has been to perform the curing cycle using the so-called "stepped cure". During the stepped cure process, the lay-up is heated to a very precise temperature and held there for a certain time in an effort to effect a jelling of the resin without excessive bleeding or squeeze-out. Typically, a compacting action, using a pressure source external to the vacuum system, had to be started at a very precise moment during the curing cycle to avoid both excessive bleeding of resin from the lay-up and/or a consolidation of the resinous material within the lay-up without compaction.
Accordingly, there has been a need for a novel method for the curing of composites through which very precise control can be maintained of the resin-to-reinforcement ratio within the lay-up, by altogether preventing the squeeze-out of resin from the lay-up during the curinng cycle. It would be desirable if such a novel system could utilize highly elastic films which could be easily tailored to the particular shape of the composite structure to be formed. Moreover, the novel system must also provide for the evacuation of entrapped air within the lay-up, as well as for the removal of volatiles, when required. The present invention fulfills these needs and provides other related advantages.